Digital Cinema projectors have to project images according to the DCI (Digital Cinema Initiatives) standard. In this standard, the color gamut of the native (colors on screen without any electronic color correction) and the electronically color corrected gamut projected by a projection device is described.
Colors can be defined with respect to standards, as the standards defined by DCI. In DCI standards, colors are defined with respect to CIE 1931 color spaces, quantitatively relating the colors as defined by their wavelengths to the physiological perception of colors in human vision. The CIE 1931 color space is shown in FIG. 1.
FIG. 1 shows the requirements of a color gamut defined in the CIE 1931 color space to comply with DCI requirements. DCI standards require the primary colors red, green and blue to be respectively defined within the tolerance boxes 10, 20, 30 of FIG. 1 and the secondary colors yellow, cyan and magenta, to be respectively enclosed by the reference points 15, 25, 35.
Another DCI requirement is to electronically correct the native white to the target DCI white point, 50. Electronic correction of a primary color in a projection device is performed at the level of the light modulation device. State of the art projection systems comprise three light modulation devices, each being dedicated to a color channel, or a primary color channel. The light modulation device can be a digital micro mirror device (DMD), whose range of operation is reduced when being electronically controlled.
One of the drawbacks of electronically corrected images is that the contrast ratio is lowered. The black level intensity stays status quo but the white level intensity drops when aiming to a dedicated white color point like the DCI white target point. If for example in the spectrum of the light source green light is dominating, this effect of loss of contrast can be dramatic. A second drawback of electronically corrected images is the loss in bit depth (grey scales).
A native white color point which is far from the target white point can lead to a significant loss of contrast ratio and of bits and thereby have a negative impact on image quality.
A third drawback of illuminating the light modulator with light that is not used on screen is the cooling needed to get rid of the absorbed amount of energy. Less light on the modulator means less cooling requirements as well.
These three reasons should make projection manufacturers realize that less electronic correction is needed; thereby resulting in a better image quality and less cooling requirements.
In the current state of the art of lamp projectors (Xenon lamp), a notch filter is used to obtain the color gamut as described in the DCI standard. Further on, typical light losses due to the electronic correction for correcting the native DCI white point to color corrected white point with a state of the art xenon lamp projector is about 4%. The reason for this low light loss is because the xenon light native white point is close to the target DCI color corrected white point. FIG. 2a shows the spectrum 220 of a typical xenon light and the transmission 210 of a typical filter used in association to the xenon light.
FIG. 1 further shows a resulting color gamut when a xenon illumination is used together with a notch filter. With the notch filter, the native red and green color are situated in the DCI tolerance boxes. The position of the native white with a notch filter is shown with reference 40. Mostly blue light has to be eliminated to match the target DCI target white 50, which implies little losses in contrast ratio and light output.
However, the effect of a notch filter and electronic correction for a laser phosphor light source is very different as for a xenon light source.
State of the art digital projection systems, use solid state light sources, in particular lasers and LEDs, usually arranged in an array, i.e. a laser array, to form a light source and provide the required power. Lasers are usually preferred with respect to LEDs due to the smaller étendue of laser light.
Laser-based solid state projectors could be classified in two main categories:                Full laser projection systems (using direct red, green and blue lasers)        Laser phosphor projection systems (using blue laser to excite a wavelength convertor material to generate the three primaries).        
Currently the full laser projectors are typically ultra-bright projectors aimed at the niche market of digital cinema (DC). Laser phosphor projectors mainly have a light output under 12K lumens and therefore are sold in the markets outside digital cinema. However, recent improvements in the phosphor technology allow laser phosphor projectors to achieve even brightness levels up to 20K lumens and possibly higher.
High brightness and colour performance are important because a digital cinema projector has to project images according to the DCI standard.
Laser phosphor projection systems use a single blue laser source, comprising an array of lasers, for simultaneously generating the blue primary color on screen and for exciting a phosphor. The red and the green primary colors are deduced from the phosphor light beam. As the spectrum of a laser phosphor source is very different from that of a xenon lamp spectrum, another type of notch filter is needed to achieve the DCI native standard white point. Blue laser arrays for the blue laser source are preferred instead of blue LED's for the phosphor excitation due to the smaller etendue of laser light
The main drawback of a laser phosphor light source with blue lasers and a phosphor is the lack of red light and the excess of green and yellow light in the spectrum of the projected image, as shown in spectrum 230 of FIG. 2b, which is the spectrum of the projected image after the color splitting and recombination taking place in an imaging module. A notch filter for removing yellow light in excess is usually used. The spectrum of the projected image using the laser phosphor light source with the yellow notch filter is shown in the spectrum 240 of FIG. 2b. 
Because of the excess of blue and green light, this light has to be electronically removed to match the native white point to the DCI white point. The typical loss of light and contrast ratio is 20% to 30%. This is much higher than for a xenon lamp projector.
Conventional systems will use a typical notch filter and color correction to be DCI compliant for the color gamut. The result of this will be a loss of contrast ratio and bit depth or in general image quality and a higher then needed energy load on the light modulator, thereby generating additional heat in and around the light modulators.
Another problem typical for laser phosphor illuminated systems is the change of the white color point (white color shift) with dimming of the blue lasers for the phosphor excitation. The white point shift is shown by the dots 45 of FIG. 1. The dimming of the blue lasers, which can be due to ageing of the lasers, increases the efficiency of the phosphor with lower blue power on the phosphor. The ratio of yellow to blue light increases with lower blue power and as a consequence the white becomes more yellowish, and the white point 45 shifts. A similar behaviour will occur during the lifetime of the laser phosphor light source.
DCI is used here as an important driver of what is generically called WCG (Wide Color Gamut). Digital Cinema has always been a frontrunner in this requirement of wide color gamut because of the typical link to film-based projectors with their specific color gamut performance.